initialDestinationId property
The destination shown in the detail pane until the user picks one.
Defaults to the first destination in sections, so the detail pane is never empty (iPad, Android and Chrome all open on a page), unless emptyDetailBuilder is set. The page only fills an empty detail pane: when the window narrows to one pane, the list shows, not this page.
Implementation
final String? initialDestinationId;